Saint-Gobain Performance Plastics

Saint-Gobain Performance Plastics

June 10, 2010 05:20 ET

Enhancing the Benefits of EPAS

Electric Power Assisted Steering (EPAS) Is Growing in Popularity, Not Least Because It Cuts Costs and Helps to Reduce Vehicle Weight. But, Says Chris Needes of Saint-Gobain Performance Plastics, Many Manufacturers of EPAS Systems Are Missing Opportunities to Maximise the Benefits of Their Systems.

BRISTOL, UNITED KINGDOM--(Marketwire - June 10, 2010) - After safety, the most important concern for car manufacturers today is vehicle weight since lower weight means lower fuel consumption and, therefore, improved environmental performance. The drive to find ways of saving weight has led to conventional hydraulic power steering systems being replaced by lighter and more cost-effective EPAS systems.

The current generation of EPAS systems undoubtedly delivers significant weight savings, while also providing a good driver interface and low noise levels, but there are now opportunities to further enhance these benefits. For example, tolerance rings and self-lubricating low-friction plain bearings can be used in many EPAS applications where the alternatives would be bulkier, heavier and more costly.

Tolerance rings are radially sprung steel rings that are press fitted between two mating components, such as a motor shaft and a take-off pulley. They are essentially a spring fastener that can compensate for thermal expansion, reduce noise and vibration, offer predictable and repeatable slip, and provide overload protection.

In EPAS systems, there are many possible applications for tolerance rings. Some manufacturers are already using them as a slipping element in steering column locks, allowing the locks to withstand the 100 Nm force prescribed by European legislation without the need for heavy-duty construction.

Used to mount the drive gear in a column-mounted EPAS, a tolerance ring gives effective torque control by acting as a "fuse", while a tolerance ring built into a steering column will provide the axial resistance necessary to ensure the controlled collapse of the column if the vehicle is involved in a collision.

Tolerance rings provide a simple and effective way of mounting all of the bearings, gears and pulleys used in EPAS systems. One application that is, however, less obvious is the mounting of sensors where conventional press fitting produces a high breakage rate, and it is difficult to achieve accurate positioning and alignment if adhesive is used. A tolerance ring around the magnet provides a solution and allows the assembly to be press fitted without risk of damage, since the fitting force is controlled by the design of the ring, rather than by component tolerances.

In EPAS systems, tolerance rings are complemented by self-lubricated plain bearings. Bearings of this type can be used in virtually all EPAS applications where conventional plain and roller element bearings are used and, in addition to reducing cost and weight, they also offer space savings, noise and vibration reduction, high load capacity and long life.

Used as liner on the inside of the steering yoke, for example, self-lubricated plain bearings give a smoother feel to the steering system because of their low friction properties. To reduce weight, many steering yokes are now made from aluminium. This means that, when ordinary steel bearings are used, large clearances are needed to allow for the difference in the rates of thermal expansion between aluminium and steel. Self-lubricated plain bearings can, however, be supplied with an aluminium base, thereby eliminating differential expansion. As a result, much smaller clearances are possible, leading to reductions in vibration and noise.

Other EPAS-related applications for self-lubricated plain bearings include the replacement of needle bearings within pinion gears, worm drives, and Cardan joints, as well as providing a low-friction surface in the steering column itself for height adjustment. Self-lubricating plain bearings are able to handle much higher loads than rolling-element bearings, and, because of their low friction they can also allow smaller assist motors to be used in EPAS systems, once again leading to savings in weight and cost.

Of course, novel components are of no value if they compromise the life or reliability of the systems in which they are used. However, both tolerance rings and self-lubricated plain bearings are at least as durable as the conventional components they replace and simulated long-duration cycle tests carried out by Saint-Gobian Performance Plastics on its Rencol tolerance rings and Norglide self-lubricated plain bearings have shown that their lives can be expected to comfortably exceed 20 years.

EPAS systems are making a major contribution toward the development of cars that are more efficient, more environmentally friendly and more comfortable to drive. Getting the most from EPAS systems, however, involves careful design and careful choice of components. Conventional fixing systems and bearings will get the job done but, as we have seen, there is a lot to be gained with innovative products like tolerance rings and self-lubricating plain bearings.

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